The following are definitions of various terms as used herein for better understanding of the inventive system for improving power factor in an AC power system:                1) An “AC power system” connotes herein a source of power and an electrical load that is interconnected by at least one power conveying line. An AC power system may provide power to a single building, for instance, or may provide power to many buildings, as in a power distribution system.        2) A “power conveying line” connotes herein a circuit comprising a set of electrical conductors for conveying power between two points and includes, but is not limited to, a power transmission line or a power distribution line. A power conveying line may be included within a power generation facility itself, or may be used for power transmission or distribution, or may be included within a facility of an end-user of power.        3) A “power transmission line” connotes herein an electrical power conveying line that connects one or more electrical sources (e.g., power conveying line, generator, or electrical substation) to one or more electrical loads such as an electrical substation. A power transmission line typically operates near or above 100 kVolts        4) A “power distribution line,” as used herein, connects an electrical substation to individual users, often with local voltage step-down transformers, or an end-user load. The end-user load may be within a single industrial or commercial site, such as a steel mill or other manufacturing facility or a school, by way of example, and typically operate near or below 65 kVolts.        5) “Medium voltage” connotes herein approximately the range from 600 Volts to 70 kVolts, in accordance with electrical power industry usage.        6) “High voltage” connotes herein voltages above 70 kVolts, in accordance with electrical power industry usage.        7) “Power Factor” is defined as the cosine of the angular displacement between the electrical kiloWatt vector and kilovVoltAmpere vectors of any AC electrical circuit. Power factor can be represented as the cosine; i.e., 0.8, etc., or as the cosine multiplied by 100 and represented in percent; i.e., 80%, etc. Power Factor can be expressed as a positive or negative number. A positive power factor indicates that the cosine between the kilowatt vector and the kilovoltampere vector is influenced by electrical inductance. A negative power factor indicates the cosine between the kilowatt vector and kilovoltampere vector is influenced by electrical capacitance. The optimum power factor for any AC electrical power system is 1.0, the equivalent of 100%. During a time period when an AC power system operates at a power factor of 100%, all of the delivered energy is in the form of kilowatts.        8) As used herein, “anti-parallel connected” unidirectional cold-cathode field-emission electron tubes refers to a pair of the foregoing type of electron tubes that are connected in anti-parallel or inverse-parallel manner. Thus, the pair of the foregoing type of electron tubes is connected in parallel but with their polarities reversed allowing operation in AC circuits.        9) As used herein, the term “controlled” in relation to control of current level refers to any of (a) grid-controlled conduction as implemented by one or more electron tubes, (b) control terminal-controlled conduction implemented by semiconductor device(s) via one or more control electrodes, or (c) controlled conduction implemented by other device(s) such as saturable reactors that have one or more control elements to control conduction, where, for each of (a)-(c), controlled conduction connotes controlling the level of current through the tube(s) or device(s) in an analog, continuously variable fashion.        10) A “control terminal” as used herein connotes (a) a control electrode of an Insulated Gate Bipolar Transistor (IGBT) or Field-Effect Transistor (FET) or other semiconductor device that regulates current therethrough in a continuously variable manner, or (b) a terminal of a control winding of a saturable reactor. Further, “control terminal” connotes herein a generic term that includes a grid for an electron tube, a base for an IGBT or a gate for an FET, for instance.        11) The phrase “bidirectional circuit with controlled current conduction” connotes a generic phrase that includes a “bidirectional electron tube circuit,” and also a bidirectional circuit that includes any of (a) an Insulated Gate Bipolar Transistor (IGBT) or Field-Effect Transistor (FET) or other semiconductor device that regulates current therethrough in a dynamically adjustable, continuously variable manner, (b) a terminal of a control winding of a saturable reactor, or (c) a rheostat (i.e., adjustable resistor).        12) The phrase “continuously variable” used in connection with current regulation, for instance, connotes the ability to have a non-broken (i.e., continuous) range of values of current, as opposed to having only non-continuous, discrete values.        
For maximum efficiency in an AC power system, it is desired to improve the power factor in the system. Power factor is a dimensionless number representing the ratio of real power, expressed in kilowatts, flowing to an electrical load to the apparent power being provided, which includes any capacitive or inductive components in addition to any real power. The power factor can be expressed as between 0 (or 0%) for a pure inductive load and −1 (or −100%) for a pure kilowatt reverse power delivery, or as between 0 (or 0%) for a pure capacitive load and 1 (or 100%) for a pure kilowatt load. A power factor of 1 or 100% is considered ideal. Improving power factor may be typically accomplished by decreasing Volt-Ampere Reactance (“VAR”) in the system. VAR is the unit used to express reactive power in an AC power system. Reactive power exists in an AC circuit when the current and voltage are not changing at the same time (out of phase). VARs may be considered as either the imaginary part of apparent power, or as the power flowing into a reactance load, where voltage and current are specified in Volts and Amperes; the two definitions are equivalent. Power Factor is also the cosine of the angle between the voltage and electrical current flowing in a circuit. Volt Amperes Power is the hypotenuse of a triangle constructed using the Real (kilowatt) Power as its base and the Volt Amperes Reactive Power (Vars) as the vertical side that is oriented 90 degrees to the base.
In the prior art, a Static VAR Compensator (SVC) or a Static Synchronous Compensator (STATCOM) can be used to reduce VAR in an AC power system by coupling or decoupling one or more reactive impedance elements to a power conveying line for a load. This may occur, for instance, by the process of connecting or disconnecting one or more capacitors or inductors between a power conveying line for a load and ground by one or more respective semiconductor or mechanical switches.
Drawbacks of using the foregoing semiconductor or mechanical switches for VAR reduction include limitations on the number of switching operations for mechanical switches before necessary replacement of switching contacts. This adds significant maintenance and replacement costs for the switching contacts.
Additionally, because the mentioned semiconductor or mechanical switches are limited in their voltage withstand capability to well below typical power conveying line voltage levels, the switches must be controlled so as to switch at, or very near to, zero current line crossings. Operation at higher-than-nominal line voltages can lead to serious damage and shortening of operation life of the switches.
A further drawback of using the mentioned semiconductor or mechanical switches for VAR reduction is the following: If the voltage on the power conveying line exceeds the voltage ratings of the switches or reactive impedance elements, then, in accordance with electrical industry practices, either the SVC or STATCOM will utilize a ferrous-core electrical transformer placed between the power conveying line, on the one hand, and the switches and reactive impedance elements, on the other hand. Such a ferrous-core electrical transformer is required to reduce the source voltage to a level that is tolerable to the switches and reactive impedance elements. Typically, this occurs where power conveying line voltages reach or exceed approximately 65 kVolts, which is standard on a power transmission line or many power distribution lines.
It would be desirable to avoid the use of mechanical or semiconductor switches, for increased reliability. Additionally, in systems of sufficiently high voltage operation, it would be especially desirable to avoid the need for a ferrous-core electrical transformer, which is expensive, bulky, occupies valuable floor space. Additionally, a ferrous-core electrical transformer of substantial size requires years to have manufactured, and an installation requiring such a ferrous-core electrical transformer will encounter significant capital costs.